Method for starting continuous metal casting operation

ABSTRACT

The process is characterized in that, before starting casting, an initialization position is determined for the stopper rod control actuator when the stopper rod is resting on its seat under its own weight and after having filled the tundish equipped with this stopper rod, the actuator is operated to place it in a controlled superclosed position and, to start casting, the actuator is driven according to an imposed movement versus time law, time (t1) for start of casting being determined from this law and the control actuator continues to be driven in opening direction to enable the metal to flow into the ingot mould.

BACKGROUND OF THE INVENTION

This invention concerns the continuous casting of metals, especiallysteel, and more particularly a process for starting casting from asituation where the casting installation is ready to receive the moltenmetal contained in a transport receptacle such as a casting ladle.

Such an installation conventionally includes a tundish equipped with anozzle and an ingot mould. The tundish also includes blocking means,called a stopper rod, to block the nozzle and adjust the flow rate ofthe molten metal during casting.

Before starting casting, a dummy bar is placed in the installation; thisdummy bar includes a dummy bar head which is inserted into the ingotmould to temporarily close the mould at the start of casting, and thestopper rod is placed in a closed position.

To start casting, the metal in the ladle is poured into the tundish.

Then, the stopper rod is opened so that the metal can fill the ingotmould by flowing through the nozzle. When the metal reaches apredetermined level in the ingot mould, the dummy bar is drivendownwards to start the extraction of the cast product at least partiallysolidified as it comes into contact with the cooled walls of the ingotmould.

One of the problems is to define the time when extraction will bestarted especially on account of the level required in the ingot mouldand the time required to obtain a sufficiently solidified product beforestarting extraction. Now, the arrival of the metal at a required levelin the ingot mould depends on the flow rate of the metal in the nozzleand therefore especially on the stopper rod opening position.

Consequently, to automate startup, it is already known to use a leveldetector to detect the arrival of the metal at the required level in theingot mould and to control the start of the extraction by this detector.

In addition, it is also known to use a level detector placed on theingot mould to control the flow rate or the extraction speed duringcasting so as to conserve metal level more or less constant in the ingotmould throughout the casting process.

However, these detectors can only be placed at the top of the ingotmould. Also, their detection distance is conventionally low and they arepositioned so as to measure the variations in level in the vicinity ofthe reference level during casting. Therefore, they only detect themetal in the ingot mould when the metal is approaching the referencelevel. Thus, during almost the complete ingot mould filling time, thelevel of the metal cannot be controlled. In addition, when the detectorcan finally detect the presence of the metal, and therefore commandstartup of extraction, a certain time is required before extraction isstabilized and level of metal may pass well beyond the reference level.This can be avoided in part by simultaneously commanding the closing ofthe stopper rod to reduce the supply rate. However, the stopper rodreaction time, positioning of which is ensured by an actuator, cannot besufficiently reduced to totally avoid the above mentioned problem. Also,the inertia of the flowing metal and that of the stopper rod controlmeans leads to fluctuations in the level which may last for a certaintime before regulation, and therefore the level, are stabilized andcasting becomes uniform.

Another problem which arises is the ability to determine the effectivecasting start time, that is, the time when the metal contained in thetundish starts to flow when the opening of the stopper rod is commanded.This problem is also related to that of being able to control the risein level in the ingot mould, this level being undetectable during amajor part of the filling operation as we saw previously. The only meansfor controlling this rise in level is therefore to act on the flow rateof the metal flowing from the tundish which depends on the exactposition of the stopper rod. However, the position of the stopper rod isconventionally determined by a measuring device located on the stopperrod control means and not on the stopper rod itself. The result is thatthe indication given by these measuring means is not exactlyrepresentative of the position of the stopper rod itself this being duemainly to the inevitable plays in the mechanical means connecting thestopper rod to its control means. Therefore, not only is there a timelag between the stopper rod opening command and the start of realopening and therefore start of metal flow but also consequentlyindication of stopper rod position does not exactly reflect itseffective position which determines the metal flow rate. However, it isonly by knowledge of the exact casting start time, and of the flow rate,that the level in the ingot mould during filling can be accuratelydetermined.

This problem is especially troublesome for the continuous castingprocess between rolls as determining the exact time of start ofextraction is of prime importance for this technique. It is thereforenecessary to accurately know the exact time of start of flow and thefilling flow rate especially as, in this technique, the filling timebetween start of flow and start of extraction is very short.

SUMMARY OF THE INVENTION

The purpose of this invention is to solve the various above mentionedproblems and in particular to allow effective casting start time andcasting flow rate during the ingot mould filling phase to be accuratelydetermined.

With these targets in mind, the subject of the invention is a startingprocess for a continuous metal casting operation in a castinginstallation including a tundish which includes an outlet hole which canbe blocked by a stopper rod bearing on a stopper rod seat, mechanicalconnecting means between the stopper rod and a stopper rod controlactuator, and an ingot mould receiving the metal flowing through thesaid hole.

According to the invention, this process is characterized in that,before starting casting:

a) the stopper rod is placed on its seat under the effect of its weightalone, the control cylinder being inactive and in an initializationposition defined by the position of the stopper rod,

b) the said initialization position of the control actuator isdetermined,

c) the control actuator is driven in closing direction to push thestopper rod onto its seat,

d) the tundish is filled with liquid metal,

e) the control actuator is operated to place it in a controlledsuperclosed position defined by a predetermined distance of the positionof control actuator in relation to initialization position,

and, to start casting:

f) the control actuator is driven in opening direction according to animposed and predefined control actuator movement versus time law,casting start time being determined from this law, by calculating, withthis law, the time taken by the control actuator to move fromsuperclosed position to the initialization position,

g) and operation of the control actuator is continued in the openingdirection to allow the metal to flow into the ingot mould.

As will be better understood later, the process according to theinvention enables the time when the molten metal will start to flowbetween the stopper rod and its seat to be accurately determined.

This time is therefore exactly the moment when the stopper rod leavesits seat.

Theoretically, it would be sufficient to hold the stopper rod exactly inthis position and to start to move it upwards to move it away from itsseat, the exact time of start of movement defining casting start time.

However, in practice, this is impossible. Indeed, because of theinevitable plays existing in the mechanical connecting means between thestopper rod and the control actuator, and because of the thrust exertedon the stopper rod by the liquid metal contained in the tundish, it isclear that, even if the control actuator was held in a fixed so-calledinitialization position where the stopper rod just rests on its seat,the exact position of the stopper rod will vary when the tundish isfilled with molten metal, especially due to the taking up of themechanical plays and to the expansion phenomena.

The result would be that the tightness of the stopper rod on its seatwould no longer be ensured and unwanted molten metal flows could occurbefore the tundish is filled.

To avoid this, conventionally according to the earlier technique, theoperator operates the stopper rod control actuator before the tundishstarts to fill, to forcibly press the stopper rod onto its seat. It isthen practically impossible for the operator to exactly know when thestopper rod will be in the tightness limit position on its seat, when hewill operate the control actuator in the opposite direction, as there isno exact correspondence between the position of the control actuator andthe position of the stopper rod.

The principle of the invention is in fact to artificially reestablishthis correspondence starting with the idea that, although there is noexact correspondence between the respective positions of the stopper rodand the control actuator when the control actuator is driven in onedirection and then in another, this correspondence is howeverreestablished if we consider movement in only one direction, that is inthe opening direction.

For this, according to the invention, an exactly measurable andtherefore reproducible position of the control actuator is defined,called the controlled superclosed position, together with a controlactuator movement law in the opening direction, that is corresponding toan upward movement of the stopper rod.

The controlled superclosed position is defined by a preestablisheddistance considered from the control actuator position which just causesthe stopper rod to separate from its seat, that is the initializationposition.

It can be seen that this initialization position of the control actuatoris not defined by the operator or by any action on the said actuator butthat it results only from the forces of gravity exerted on theinstallation and, in particular, on the stopper rod. It is thereforeonly the contact of the stopper rod on its seat, under its own weight,which defines the initialization position of the control actuator. Itcan therefore be seen that, when the initialization position isdetermined, it is the stopper rod which fixes the position of thecontrol actuator whereas, during casting, it is obviously the controlactuator which fixes the position of the stopper rod.

The actuator movement versus time control law is defined experimentally,according to the characteristics of the casting installation and theprocess, so as to establish a well-defined relation between the movementof the control actuator and the movement of the stopper rod as soon as,moving upwards, the stopper rod is no longer in contact with its seat.However, before this time, there is no such well-defined relation butonly a definition of the position of the control actuator versus time,without the actual position of the stopper rod being related to that ofthe actuator.

This law will therefore enable and ensure a movement of the actuator,without there being a proportional movement of the stopper rod, thismovement of the actuator corresponding, in a way, to the releasing ofthe stresses generated by the force pressing the stopper rod onto itsseat.

Then, when the stresses are eliminated, that is from the time thestopper rod leaves its seat, the movement of the actuator leads to amovement of the stopper rod and therefore to the flow of the metalcontained in the tundish, the flow rate of the molten metal then beingcontrolled by acting on the actuator and dependent on the position ofthe said actuator.

The explanation above is intended to describe the principle of theinvention and is therefore fairly theoretical. In practice, it isobvious that the actual start of flow does not exactly correspond to thetime when the stopper rod leaves its seat, especially because thegeometry of the surfaces in contact is not ideal and that the physicalcharacteristics of the molten metal (fluidity, surface tension, etc.)enter into effect. This is why the actuator movement law is determinedexperimentally, one of the goals targeted by the invention beingessentially to be able to ensure reproducibility of the startingconditions from one casting to another.

According to a specific arrangement of the invention, the stopper rod ispushed onto its seat until the thrust force exerted by the control meansreaches a predetermined value.

Alternatively, the stopper rod is pushed onto its seat until the controlmeans reach a predetermined position.

Whatever the case, the thrust exerted on the stopper rod before theintroduction of the molten metal into the tundish must be sufficient toguarantee perfect tightness of the stopper rod on its seat without riskof this tightness being disturbed during the filling of the tundish.However, this superclosed position will be located beyond the controlledsuperclosed position in the closing direction.

According to another additional arrangement, when casting is started andafter the control means have been set to initialization position,automatic opening of the stopper rod is continued according to animposed opening law up to the so-called filling position. This fillingposition is maintained throughout the filling of the ingot mould. Thisarrangement in fact enables the filling of the ingot mould to be ensuredunder a controlled flow rate so that the rising of the liquid metal inthe ingot mould is achieved as calmly as possible and the conventionallyknown level regulation can be brought into play smoothly, withoutcreating surges, when the metal level in the ingot mould reaches a levelnear to the nominal level. This especially avoids all risks of the metaloverflowing from the ingot mould. This also ensures smooth transitionbetween the starting phase, that is until the level in the ingot mouldis more or less equal to the nominal level, and start of extraction.

According to yet another preferential arrangement, before the level ofthe metal in the ingot mould reaches the predetermined nominal castinglevel, level regulation is activated to ensure the regulation of thelevel as soon as the level of the metal reaches a level near to thenominal level. Level regulation, well known in continuous castinginstallations, is therefore set into service well before the level ofthe metal can be detected by the sensor conventionally used inregulation systems. However Regulation is saturated to prevent it fromtending to cause additional opening of the stopper rod (which normallywould be the case as the level of the metal is then well under thenormal level) . However, as the regulation circuits are alreadyoperating before the sensor detects the metal poured into the ingotmould, the regulation system acts without delay as soon as the level ofthe metal is detected. The result is that the reaction, caused by thesaid regulation when the poured in metal reaches a level near to thenominal level, is less sharp and does not generate a sudden movement ofthe stopper rod or a sudden variation in extraction speed.

Other advantages and features will appear in the description which willfollow of a starting process for a continuous steel casting installationin compliance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Refer to appended drawings on which:

FIG. 1 shows a schematic view of an ingot mould continuous steel castinginstallation,

FIG. 2 is a graph showing the measured position of the stopper rodcontrol actuator versus time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The continuous casting installation, shown on FIG. 1, during casting,includes a tundish 1, containing molten steel 2, equipped with an outlethole 3 with a nozzle 4. The outlet hole 3 can be blocked by a stopperrod 5 when it bears on its seat 6. The movements of the stopper rod aremade by a control actuator 7, connected to the stopper rod 5 bymechanical connecting means such as a lever 8 hinged so as to pivot in abearing 9.

The installation also includes, in a manner known itself, an ingot mould20 the walls of which are vigorously cooled to cool and solidify themolten metal poured into the ingot mould through the nozzle 4. At normalcasting speed, the metal at least partially solidified in the form, forexample, of a slab 21, is extracted from the ingot mould via the bottom,using extraction rolls 22 rotated by motors not shown.

The control actuator 7 is equipped with a position sensor 10 whichpermanently measures the exact position of the actuator rod. Theinstallation also includes a regulation system 11, shown schematicallyon the drawing and which is also connected to a level detector 12 todetect and measure level 23 of the metal in the ingot mould.

The regulation system 11 is also connected to a solenoid valve 13, or toequivalent control means, to control the movements of the actuator 7,and to the motors of the extraction rolls 22 to control their speed.

All these means are conventionally known in existing castinginstallations.

Note, however, that control actuator means not only a conventionalactuator including a rod movable in translation in an actuator body,such as the actuator 7 shown in FIG. 1, but also any other actuatorwhich can ensure the same stopper rod movement function.

The graph in FIG. 2 shows, as an example of the process according to theinvention, the variations in position d of the actuator 7 versus time tfrom a time before the start of the process until a normal casting speedis reached.

The plot 31 corresponds to the initialization position “0” of thecontrol actuator, that is the measured position of the actuator rod whenthe stopper rod 5 is resting under its own weight on its seat 6. Theactuator is then submitted to no pressure from the solenoid valve 13,the position of the rod being determined only by the position of thestopper rod 5. In the example shown, it is easy to understand that theweight of the stopper rod and that of the actuator rod exert downwardloads on the lever 8 and that, consequently, the inevitable plays in allthe hinge points are at the top for the hinge points 51 and 61 of thestopper rod and the actuator rod on the lever and for the hinge point ofpivot 9 of the lever on the installation.

From this position, the actuator is then driven so that its rod moves bya value d1, so as to compensate for the various plays mentioned aboveand to forcibly press the stopper rod onto its seat. This position isconserved throughout the time the tundish is filled with metal as shownon plot 32. Instead of defining, at this stage, a position of theactuator d1, an actuator supply pressure or load could also be definedas was already stated.

When the tundish is full, the actuator is then driven to move the rod tothe so-called controlled superclosing position (plot 33 in FIG. 2). Thisposition is defined by a distance d2 from the initialization position.This distance can be for example a predetermined percentage, for example3%, of the total stroke of the actuator. This distance will in practicebe determined experimentally so that it will not be too high but,however, sufficient so that the stopper rod remains adequately pressedonto its seat so that no play appears at the various hinge points.

This position will then be considered as the actuator control startpoint under the already mentioned imposed opening law.

This law is shown in FIG. 2 by plot 34. This law, as represented, fixingthe movement of the actuator rod versus time, is linear. This is howevernot mandatory, the curve representative of the said law could deviatesomewhat from a straight line according to the kinematics of theconnecting means between the actuator and the stopper rod and also theingot mould filling rate conditions as will be seen later.

Therefore, knowing the actuator movement law, the distance d2 and thetime to at which actuator movement is controlled in compliance with thislaw, the accurate starting time t1 will be determined by calculation asbeing time to+Δt, Δt being the time taken for the actuator rod to moveover the distance d2. At this time, the movement of the actuator willhave released the pressure exerted by the stopper rod on its seat duringthe controlled superclosing phase and will have thus taken up all hingepoint plays in the direction opposite to the one produced during thepressurization of the actuator before filling the tundish. The actuator,lever and stopper rod assembly is then more or less in the samesituation as during initialization because the forces exerted arepractically the same as those exerted during initialization, the onlydifference being that it is the actuator which pulls the stopper rodupwards whereas, during initialization, it is the stopper rod whichretains the actuator.

From time t1, the flow outlet hole therefore opens gradually, theopening operation being controlled by the movement of the actuator, thiscontinuous movement being made under the control of the imposed law upto a point defined by distance d3, this distance d3 being determined soas to correspond to a given opening of the stopper rod. It may bedifferent from the maximum opening provided for the normal casting speedwhich will only be reached after extraction has been started. From timeti, the metal contained in the tundish therefore begins to flow into theingot mould at a flow rate determined by the opening of the stopper rod,that is gradually increasing until the actuator reaches position d3,then stabilizing at an imposed value whilst the filling of the ingotmould continues (plot 35).

During the time the ingot mould is filled, before extraction starts, themetal flow rate can therefore be different from the nominal flow rateunder which the metal will flow from the tundish into the ingot mouldafter the startup of the extraction rolls. It is only when the level ofmetal in the ingot mould reaches a level near to level detector 12 thatlevel regulation, of a type known itself, takes over to control theactuator 7, and possibly the speed of the extractor rolls 22, to adaptthe flow rate to the extraction speed so as to conserve a more or lessconstant level of metal in the ingot mould as is well known.

The invention is not limited to the starting process described hereabove only as an example. In particular, the process according to theinvention could be advantageously used in continuous castinginstallations between rolls.

Also, instead of using the sensor 10 directly placed on the actuator andmeasuring the position of the rod, the position measurements could bemade by any other measuring means suitable to accurately determine theposition of the stopper rod control means.

What is claimed is:
 1. A starting process for a continuous metal castingoperation in a casting installation including a tundish with an outlethole which can be blocked by a stopper rod which bears on a stopper rodseat, mechanical connecting means between the stopper rod and a stopperrod movement control actuator, and an ingot mould receiving the metalflowing through the said outlet hole, characterized in that, beforestarting casting: a) the stopper rod is placed on its seat under theeffect of only its own weight, the control actuator being inactive andin an initialization position defined by the position of the stopperrod, b) the said initialization position of the control actuator isdetermined, c) the control actuator is then driven in the closingdirection to force the stopper rod onto its seat until a thrust forceexerted on the control actuator reaches a predetermined value, thecontrol actuator being then in a superclosed position located a firstdistance from the initialization position, d) the tundish is filled withliquid metal, e) the control actuator is driven to place the controlactuator in a controlled superclosed position defined by a predeterminedsecond distance of the position of the control actuator in relation tothe initialization position, the predetermined second distance beingless than the first distance, and, to start casting: f) the controlactuator is driven in the opening direction according to an imposedcontrol actuator movement versus time law, the time for start of castingbeing determined from this law by calculating, with this law, the timetaken by the control actuator to move from the controlled superclosedposition to the initialization position, g) and the control actuatorcontinues to be driven in an opening direction to enable the metal toflow into the ingot mould.
 2. A starting process for a continuous metalcasting operation in a casting installation including a tundish with anoutlet hole which can be blocked by a stopper rod which bears on astopper rod seat, mechanical connecting means between the stopper rodand a stopper rod movement control actuator, and an ingot mouldreceiving the metal flowing through the said outlet hole, characterizedin that, before starting casting: a) the stopper rod is placed on itsseat under the effect of only its own weight, the control actuator beinginactive and in an initialization position defined by the position ofthe stopper rod, b) the said initialization position of the controlactuator is determined, c) the control actuator is then driven in theclosing direction to push the stopper rod onto its seat until thecontrol actuator reaches a predetermined superclosed position defined bya predetermined first distance in relation to the initializationpositions, d) the tundish is filled with liquid metal, e) the controlactuator is driven to place the control actuator in a controlledsuperclosed position defined by a predetermined second distance of theposition of the control actuator in relation to the initializationposition, the predetermined second distance being less than thepredetermined first distance, and, to start casting: f) the controlactuator is driven in the opening direction according to an imposedcontrol actuator movement versus time law, the time for start of castingbeing determined from this law by calculating, with this law, the timetaken by the control actuator to move from the controlled superclosedposition to the initialization position, and the control actuatorcontinues to be driven in an opening direction to enable the metal toflow into the ingot mould.
 3. Process in accordance with claim 1,characterized in that, during the starting of the casting and after thecontrol actuator has been set to the initialization position, thestopper rod opening is continued up to a filling position located belowa full opening position, this filling position being maintained duringthe filling of the ingot mould.
 4. Process in accordance with claim 3,characterized in that, before the level of the metal in the ingot mouldreaches a nominal predetermined casting level, a level regulation systemis activated which controls the level as soon as the level of the metalreaches a level near the nominal level.
 5. Process in accordance withclaim 2, characterized in that, during the sting of the casting andafter the control actuator has been set to the initialization position,the stopper rod opening is continued up to a filling position locatedbelow a fill opening position, this filling position being maintainedduring the filling of the ingot mould.
 6. Process in accordance withclaim 5, characterized in that, before the level of the metal in theingot mould reaches a nominal predetermined casting level, a levelregulation system is activated which controls the level as soon as thelevel of the metal reaches a level near to the nominal level.